1. Field of the Invention
The present invention relates to an improvement in performance of an electric power steering apparatus for assisting a force required for a steering operation of a vehicle with the aid of a rotating force of an electric motor.
2. Description of the Related Art
As a power steering apparatus of a vehicle used for assisting a steering force, an electric power steering apparatus constructed so as to assist a driver""s steering force to a steering wheel by using a rotating force of an electric motor (hereinafter referred to as a xe2x80x9cmotorxe2x80x9d) is known.
As an example of this kind of electric power steering apparatus, a power steering apparatus adopting a rack and pinion movement converting mechanism for converting movement into linear movement is disclosed in Japanese Patent Publication (KOUKOKU) No. Hei.7-25311.
FIG. 1 is an enlarged sectional view showing a conventional rack and pinion movement converting mechanism. In this mechanism, pinion teeth 86a are formed on an outer peripheral surface of an end portion of a pinion shaft 86. A flat portion is formed in the vicinity of one end of a rack shaft 81 in an axial length direction, and rack teeth 81a are formed over the whole length of the flat portion. The pinion teeth 86a and the rack teeth 81a are engaged with one another so that rotation movement is converted into linear movement. Contrivance of keeping a steering force constant in spite of an electric motor with a relatively low rotating output is made in such a manner that the rack teeth 81a are engraved such that a gear ratio to the pinion teeth 86a becomes small at a position where a vehicle moves straight, and the gear ratio becomes large at a position where a steering angle is large.
In the above rack and pinion movement converting mechanism, the rack shaft 81 is pressed by an energizing force of a coil spring 89 abutting on one end of a pressing member 80 in an almost cylindrical member, so that engagement between the rack teeth 81a and the pinion teeth 86a is kept. By this engagement, rotation of the pinion shaft 86 is converted into linear movement of the rack shaft 81 in the axial length direction, and the pressing member 81 slides on the outer peripheral surface of the rack shaft 81. Thus, in this structure, abrasion of the sliding surface is remarkable, so that the pressing force (energizing force) of the coil spring 89 is limited. Thus, the electric power steering apparatus, such as a pinion assist type electric power steering apparatus in which a relatively large torque is applied to a pinion shaft, has such a defect that a necessary pressing force can not be obtained and an abnormal sound such as a rattle sound is generated from the periphery of the engagement surface of the pinion teeth 86a and the rack teeth 81a. 
In the electric power steering apparatus, in general, a rotation shaft of a steering wheel is connected to a pinion shaft of a rack and pinion movement converting mechanism through a joint. The electric power steering apparatus includes a column type power steering apparatus in which a motor and a speed reduction mechanism of the motor are positioned nearer to a steering wheel side than the joint, and a pinion type electric power steering apparatus in which a motor and a speed reduction mechanism of the motor is positioned nearer to a rack and pinion mechanism side than the joint.
When the output of an electric power steering apparatus is increased, there occurs a problem that the size of a motor is limited due to a layout and the like, and the output of the motor is also limited. In order to solve the above problem, there is proposed a method in which the efficiency of a mechanical system, especially a rack and pinion movement converting mechanism is raised to effectively use the efficiency of the motor. As this method, in the rack and pinion movement converting mechanism, a technique (hereinafter referred to as a xe2x80x9crolling type rack and pinion movement converting mechanismxe2x80x9d) using a rotatable roller as a pressure pad portion to press a rack shaft against a pinion shaft has been devised. According to the rolling type rack and pinion movement converting mechanism, in the case where a significant load is applied between the rack shaft and the pinion shaft, the mechanical efficiency can be greatly improved.
However, in the case where the rolling type rack and pinion movement converting mechanism is combined with the column type electric power steering apparatus or the pinion type electric power steering apparatus, since a motor does not respond to a kickback from a road surface, as compared with another conventional electric power steering apparatus, an excessive load is applied to the rack and pinion mechanism.
Further, in this case, since attenuation of a kickback force is inferior to a conventional sliding system since the rolling occurs on the sliding surface, there is a problem that a rattle sound is apt to be generated.
Furthermore, in this case, another object of introducing the rolling type rack and pinion movement converting mechanism is to improve a wheel returning property and an on-center feeling by reduction of a frictional force. However, since an influence of a frictional force of a motor is greater than an improvement by the rolling type rack and pinion movement converting mechanism, there is a problem that an effect has not been obtained to such a degree that the object is achieved.
The present invention has been made in view of the foregoing circumstances, and an object of the present invention is to provide an electric power steering apparatus which can prevent an excessive load from being generated in a rack and pinion movement converting mechanism, can suppress generation of a rattle sound, and can lessen an influence of frictional force of a motor.
The present invention relates to an electric power steering apparatus comprising a rack and pinion movement converting mechanism for converting rotation movement of a steering wheel into linear movement by engagement between a pinion shaft and a rack shaft, and a motor for assisting a driver""s steering force to the steering wheel, and the rack and pinion movement converting mechanism is a rolling type rack and pinion movement converting mechanism, and an elastic body is inserted in a mechanism passage from the pinion shaft to an output shaft of the motor, so that the object of the present invention is achieved.
Moreover, the electric power steering apparatus comprises a joint for coupling a rotation shaft of the steering wheel to the pinion shaft of the rack and pinion movement converting mechanism, and is such a column type power steering apparatus that the motor and a speed reduction mechanism of the motor are disposed between the joint and the steering wheel, so that the object of the present invention is effectively achieved.
Moreover, the electric power steering apparatus comprises a joint for coupling a rotation shaft of the steering wheel to the pinion shaft of the rack and pinion movement converting mechanism, and is such a pinion type power steering apparatus that the motor and a speed reduction mechanism of the motor are disposed between the joint and the pinion shaft of the rack and pinion movement converting mechanism, so that the object of the present invention is effectively achieved.
Moreover, the elastic body is inserted in the mechanism passage from the output shaft of the motor to a speed reduction mechanism of the motor, so that the object of the present invention is effectively achieved.
Further, the elastic body is inserted in a mechanism of the joint, so that the object of the present invention is more effectively achieved.
Still further, the electric power steering apparatus comprises control means for controlling the motor on the basis of a steering assist instruction value calculated by arithmetic means based on a steering torque generated at a rotation shaft of the steering wheel and a current control value calculated from a current value of the motor, and auxiliary arithmetic means for obtaining a differential value of a signal of the steering torque and adding the value to the steering assist instruction value, so that the object of the present invention is effectively achieved.